1. Field of the Invention
The present invention relates to a process for the production of dialkyl carbonates, particularly dimethyl carbonate wherein the reaction occurs simultaneously with separation of the reactants and the carbonate products. More particularly the invention relates to a process wherein methanol is reacted with urea and/or alkyl carbamate in the presence of a novel catalyst complex comprising a homogeneous organic tin compound and an electron donor oxygen atom containing compound.
2. Related Art
Dialkyl carbonates are important commercial compounds, the most important of which is dimethyl carbonate (DMC). Dimethyl carbonate is used as a methylating and carbonylating agent. It can also be used as a solvent to replace halogenated solvents such as chlorobenzene. Although the current price of dimethyl carbonate is prohibitively expensive to use as fuel additive, it could be used as an oxygenate in reformulated gasoline and an octane component. Dimethyl carbonate has a much higher oxygen content (53%) than MTBE (methyl tertiary butyl ether) or TAME (tertiary amyl methyl ether), and hence not nearly as much is needed to have the same effect. It has a RON of 130 and is less volatile than either MTBE or TAME. It has a pleasant odor and, unlike ethers, is biodegradable.
In older commercial processes dimethyl carbonate was produced from methanol and phosgene. Because of the extreme toxicity and cost of phosgene, there have been efforts to develop better, non-phosgene based processes.
In one new commercial process, dimethyl carbonate is produced from methanol, carbon monoxide, molecular oxygen and cuprous chloride via oxidative carbonylation in a two step slurry process. Such a process is disclosed in EP 0 460 735 A2. The major shortcomings of the process are the low production rate, high cost for the separation of products and reactants, formation of by-products, high recycle requirements and the need for corrosion resistant reactors and process lines.
Another new process is disclosed in EP 0 742 198 A2 and EP 0 505 374 B1 wherein dimethyl carbonate is produced through formation of methyl nitrite instead of the cupric methoxychloride noted above. The by-products are nitrogen oxides, carbon dioxide, methylformate, etc. Dimethyl carbonate in the product stream from the reactor is separated by solvent extractive distillation using dimethyl oxalate as the solvent to break the azeotropic mixture. Although the chemistry looks simple and the production rate is improved, the process is actually very complicated because of the separation of a number of the materials, balancing materials in various flow sections of the process, complicated process control and dealing with the hazardous chemical, methyl nitrite.
In another commercial process dimethyl carbonate is produced from methanol and carbon dioxide in a two step process. In the fist step cyclic carbonates are produced by reacting epoxides with carbon dioxide as disclosed in U.S. Pat. Nos. 4,786,741; 4,851,555 and 4,400,559. In the second step dimethyl carbonate is produced along with glycol by exchange reaction of cyclic carbonates with methanol. See for example Y. Okada, et al "Dimethyl Carbonate Production for Fuel Additives", ACS, Div. Fuel Chem., Preprint, 41(3), 868, 1996, and John F. Knifton, et al, "Ethylene Glycol-Dimethyl Carbonate Cogeneration", Journal of Molecular Chemistry, vol 67, pp 389-399, 1991. While the process has its advantages, the reaction rate of epoxides with carbon dioxide is slow and requires high pressure. In addition the exchange reaction of the cyclic carbonate with methanol is limited by equilibrium and methanol and dimethyl carbonate form an azeotrope making separation difficult.
It has been known that dialkyl carbonates can be prepared by reacting primary aliphatic alcohols such as methanol with urea in the presence of various heterogeneous and homogeneous catalysts such as dibutyltin dimethoxide, tetraphenyltin, etc. See for example P. Ball et al, "Synthesis of Carbonates and Polycarbonates by Reaction of Urea with Hydroxy Compounds", C1 Mol. Chem., vol 1, pp 95-108, 1984. Ammonia is a by-product and it may be recycled to urea as in the following react on sequence. ##STR1##
Carbamates are produced at a lower temperature followed by production of dialkyl carbonates at higher temperature with ammonia being produced in both steps. ##STR2##
As noted the above two reactions are reversible under reaction conditions. The order of catalytic activity of organotin compounds is R.sub.4 Sn&lt;R.sub.3 SnX&lt;&lt;R.sub.2 SnX.sub.2, wherein X=Cl, RO, RCOO, RCOS. A maximum reaction rate and minimum formation of by-products are reported for dialkyl tin (IV) compounds. For most catalysts (Lewis acids), higher catalyst activity is claimed if the reaction is carried out in the present of an appropriate cocatalyst (Lewis base). For example, the preferred cocatalyst for organic tin (IV) catalysts such as dibutyltin dimethoxide, dibutyltin oxide, etc. are triphenylphosphine and 4-dimethylaminopyridine. However, the thermal decomposition of intermediate carbamates to isocyanic acid (HNCO) or isocyanuric acid ((HNCO).sub.3) and alcohols is also facilitated by the organotin compounds such as dibutyltin dimethoxide or dibutyltin oxide employed in the synthesis of aliphatic carbamates. WO 95/17369 discloses a process for producing dialkyl carbonate such as dimethyl carbonate in two steps from alcohols and urea, utilizing the chemistry and catalysts published by P. Ball et al. In the first step, alcohol is reacted with urea to produce an alkyl carbamate. In the second step, dialkyl carbonate is produced by reacting further the alkyl carbamate with alcohol at temperatures higher than the first step. The reactions are carried out by employing an autoclave reactor. However, when methanol is reacted with methyl carbamate or urea, N-alkyl by-products such as N-methyl methyl carbamate (N-MMC) and N-alkyl urea are also produced. The dialkyl carbonate is present in the reactor in an amount between 1 to 3 weight % based on total carbamate and alcohol content of the reactor solution.